Somatic cell score (SCS) evaluations have been published inthe United States since 1994 and international evaluations havebeen available through Interbull since May 2001. The accumulateddata provides an opportunity to investigate the accuracy andstability of SCS evaluations. United States domestic evaluationsfrom January 1995 through August 2004, for 21,500 Holstein bullswere considered, over time and sequentially within bull, forchanges to the November 2004 evaluation. On average, predictedtransmitting ability (PTA) SCS increased (worsened) by 0.002from earlier evaluations to November 2004. Although bias wassmall, PTA changes were more than expected based on change inreliability. When looked at sequentially, bullsí earlierevaluations were generally lower (i.e., merit was overestimated)relative to November 2004. Differences were small, and PTA SCSincreased steadily with the addition of second-crop daughters.All 524,081 evaluations were considered pairwise providing over8,000,000 pairs of bullsí evaluations for analysis ofPTA
differences relative to change in reliability. Agreementof observed
and expected SD improved for larger changes in reliability.The
November 2004 US and Interbull PTA were matched with USand Interbull
PTA from May 2001 (US04, IB04, US01, and IB01,respectively) for
14,652 Holstein bulls. For bulls having onlyUS daughters in IB01,
correlations were similar for US01 andIB01 with US04, and IB01 with
IB04. Corresponding regressionswere all nearly 1.00. For bulls also
having nonUS daughtersin IB01, correlations with yield deviations
calculated for laterdaughters (used as source of independent data)
were higher (0.747vs. 0.714) for IB01 than for US01. For bulls with
added US daughters,correlation with US04 was also higher for IB01
than US01, showingthat inclusion of foreign data improved predictive
value ofSCS evaluations.